At present, electronic data is often encrypted for transmission in order to prevent the content thereof from being leaked.
For data encryption, two basic encryption methods, a symmetric encryption method (secret key encryption method) and an asymmetric encryption method (public key encryption method), are generally used.
The symmetric encryption method uses a common secret key for both encryption and decryption.
The asymmetric encryption method uses a pair of asymmetric keys.
More specifically, the asymmetric encryption method uses one secret key and one public key. Data encrypted by the public key can be decrypted only by the corresponding secret key.
Organizations such as companies and public offices retain confidential information and need to manage the confidential information.
However, there is a danger that an organization insider may encrypt confidential information and transmit it to others.
In this case, since the data has been encrypted, it is difficult to determine whether the transmitted data is confidential information or not.
Thus, there is a possibility that the organization may be unaware of the leakage of confidential information, if occurs.
In order to avoid this, a system needs to be established in which a confidential information manager (hereinafter, referred to as “privileged user”) of an organization can decrypt encrypted data irrespective of the intentions of a transmitter and a receiver and confirm whether the content of transmitted data includes confidential information or not.
Examples of a conventional method for the privileged user to decrypt encrypted electronic data are disclosed in PTL 1 and PTL 2.
Use of the method disclosed in PTL 1 allows the privileged user to decrypt encrypted electronic data.
However, in the method disclosed in PTL 1, it is necessary for the privileged user to retain and manage all information concerning the secret keys of users of the encryption system, making the key management work cumbersome and complicated.
The reason for the above is that the privileged user needs to mange information of individual secret keys of the users of the encryption system.
Use of the method disclosed in PTL 2 allows the privileged user to decrypt encrypted electronic data without a need to manage the secret keys of the users of the encryption system.
However, the privileged user needs to manage his own secret key. In addition, each of the encryption system users needs to manage two secret keys. Further, for electronic data encryption, each of the encryption system users needs to acquire four public keys.
The reason for the above is that since there does not exist a key for decrypting encrypted data but a pair of secret and public keys, it is necessary to create a pair of secret and public keys every time required.